Hi… Yes, there’s a separate email address for billing and payment glitches (and/or surprises):
For questions about a specific invoice or account management issues, email us at billing@fly.io.
Unlike the others, this is open to all users—and does not require an active plan.
(Some forum posters do report a multi-day wait, however.)
Fly.io employees occasionally field billing questions in the forum, but lately usually only to direct people to the above address.
Hope this helps!